1. Field of the Invention
The invention relates to a transparent article suitable for use as a window and particularly to an electrically conductive coating for windows of motor vehicles where it is necessary to remove ice or fog.
2. Background Information
Partially transparent coatings comprising a thin metal layer between two dielectric anti-reflection layers have many applications, including solar heat rejecting windows for buildings and vehicles. Electrically conductive coatings capable of heating the window to remove ice or fog are particularly desirable. Coatings having uniform, high conductivity and uniform transmittance of greater than 70% are needed for heated windshield applications.
U.K. Patent No. 1 307 642 to Asahi Glass Co. discloses an electrically heated window having a dielectric-metal-dielectric coating wherein the metal layer is silver and the dielectric layers are TiO.sub.2, SnO.sub.2, or Bi.sub.2 O.sub.3 among others.
U.S. Pat. No. 4,368,945 to Fujimori et al discloses an infrared reflecting laminated window comprising a plastic film on which is desposited a dielectric-metal-dielectric coating of tungsten oxide, silver and tungsten oxide.
U.S. Pat. No. 4,462,883 to Hart discloses a dielectric-metal-dielectric coating for glass or plastic windows comprising a layer of silver between two anti-reflection layers of SnO.sub.2 or other metal oxide. In order not to oxidize the silver layer, it is coated with a small amount of titanium or other sacrificial metal which is oxidized during reactive sputtering of the overlaying metal oxide layer. The resulting coated article has a low emissivity i.e., it transmits visible radiation but reflects a high proportion of the incident infrared radiation. A particular method for depositing coatings comprising layers of silver and a sacrificial metal oxide are described in U.S. Pat. No. 4,497,700 to Groth et al.
U.S. Pat. No. 4,337,990 to Fan et al. discloses a transparent heat mirror comprising a layer of sputtered silver sandwiched between layers of reactively sputtered titanium dioxide.
Laminated glass containing a dielectric-metal-dielectric coating of TiO.sub.2 -Ag-TiO.sub.2 has high visible transmittance, high infrared reflection and other superior optical properties. Unfortunately, titanium dioxide forms at a relatively slow rate by reactive sputtering; the rate is only about 8% of that for zinc oxide or tin oxide. However, when glass having a coating of ZnO-Ag-ZnO was laminated with a layer of plastic and glass by conventional techniques, the transmittance decreased by a surprisingly large amount. The resulting transmittance is only slightly greater than required for windshield applications and unusual care is required to produce it in commercial quantitites.
The present invention is directed to a dielectric-metal-dielectric coating having optical properties suitable for heated windshields and other applications and to a method for producing it in commercial quantities at a reasonable cost.